Placenta is a temporary organ that develops in pregnancy and joins the mother and foetus. The placenta acts as a filter. It transfers oxygen and nutrients from the mother to the foetus, and takes away carbon dioxide and waste products. The placenta is full of blood vessels. The placenta is expelled from the mother's body after the baby is born and it is no longer needed. It is sometimes called the afterbirth.

Foetoplacental circulation is the blood supply in the foetus and placenta.

Foetal membranes are the membranes surrounding the foetus.

Maternal-foetal microtransfusions are when small amounts of infected blood from the mother leak from the placenta to the baby during labour (or other disruption of the placenta).

Chorioamnionitis is inflammation of the chorion and the amnion, the membranes that surround the foetus. Chorioamnionitis is usually caused by a bacterial infection.

Mucosal lining is the moist, inner lining of some organs and body cavities (such as the nose, mouth, vagina, lungs, and stomach). Glands in the mucosa make mucous, a thick, slippery fluid. A mucosal lining is also called a mucous membrane.

Gastrointestinal (GI) tract is the tube that runs from the mouth to the anus and where we digest our food. The gastrointestinal tract begins with the mouth and then becomes the oesophagus (food pipe), stomach, duodenum, small intestine, large intestine (colon), rectum and, finally, the anus.

How and Why Does Transmission Happen?

Despite remarkable achievements in reducing vertical transmission, we do not fully understand how it happens. What we do understand, though, is that there are many factors that affect transmission.

Vertical transmission of HIV can happen before, during or after birth. Scientists have found several possible reasons for infection. Besides the mother's viral load, her low CD4 count and whether she has other infections can make it more likely.

The exposure of the baby to a mother's infected blood or other body fluids during pregnancy and delivery, as well as breastfeeding, are thought to be how transmission happens. Most transmissions happen during delivery when the baby is being born. More rarely, some transmissions happen during pregnancy before delivery. This is called in utero transmission.

This section has lots of medical words. We have explained in the table at right.

Transmission During Pregnancy (in utero)

This may happen if the placenta is damaged, making it possible for HIV-infected blood from the mother to transfer into the blood circulation of the foetus.

Chorioamnionitis, for example, has been associated with damage to the placenta and increased transmission risk of HIV.

This is thought to happen either by infected cells travelling across the placenta, or by progressive infection of different layers of the placenta until the virus reaches the foetoplacental circulation.

The reason we know that in utero transmission happens is that a proportion of HIV positive babies tested when they are a few days old already have detectable virus in their blood. Usually it takes several weeks from when someone is infected until HIV shows in the blood. The rapid progression of HIV disease in some babies has also made scientists conclude that this happens.

Having a high viral load and a low CD4 make in utero transmission more likely.

Having TB (tuberculosis) at the same time also makes it more likely and HIV makes in utero transmission of TB more likely.

During Labour and Delivery (intrapartum Transmission)

Transmission during labour and delivery is thought to happen when the baby comes into contact with infected blood and genital secretions from the mother as it passes through the birth canal.

This could happen through ascending infection from the vagina or cervix to the foetal membranes and amniotic fluid, and through absorption in the digestive tract of the baby.

Alternatively, during contractions in labour, maternal-foetal microtransfusion may occur.

Scientists know that transmission occurs during delivery because:

50 percent of babies who turn out to be infected test HIV negative in the first few days of life.

There is a rapid increase in the rate of detection of HIV in babies during the first week of life.

The way that the virus and the immune system behave in some newborn babies is similar to that of adults when they first become infected.

It is also shown by the success in preventing it happening. This includes:

Treatments that have reduced transmission risk, even when given only in labour

Delivery of the baby by Caesarean section, before labour starts.

If it takes a long time to deliver after the membranes have ruptured (waters breaking) or if there is a long labour, the risk of transmission in women not receiving antiretrovirals is increased.

A premature baby may be at higher risk of HIV transmission than a full term baby.

Breastfeeding

HIV in breast milk most likely gets through the mucosal lining of the gastrointestinal tract of infants.

The gastrointestinal tract of a young baby is immature and more easily penetrated than that of an adult. It is unclear whether damage to the intestinal tract of the baby, caused by the early introduction of other foods, particularly solid foods, could increase the risk of infection.

In the UK all HIV positive women are recommended to formula feed their babies to protect them from HIV.

The most important thing to know about vertical transmission is not how it happens, but how we can prevent it from happening. We can do this with antiretrovirals.

This article was provided by HIV i-Base. It is a part of the publication Guide to HIV, Pregnancy and Women's Health. Visit HIV i-Base's website to find out more about their activities, publications and services.

The Body is a service of Remedy Health Media, LLC, 750 3rd Avenue, 6th Floor, New York, NY 10017. The Body and its logos are trademarks of Remedy Health Media, LLC, and its subsidiaries, which owns the copyright of The Body's homepage, topic pages, page designs and HTML code. General Disclaimer: The Body is designed for educational purposes only and is not engaged in rendering medical advice or professional services. The information provided through The Body should not be used for diagnosing or treating a health problem or a disease. It is not a substitute for professional care. If you have or suspect you may have a health problem, consult your health care provider.